Microlens array and method of forming same and solid-state image pickup device and method of manufacturing same

ABSTRACT

A resist having a three-dimensional shape of a microlens array and a material layer of the microlens array are simultaneously etched under a condition by which planar patterns transferred from the resist to the material layer are larger than planar patterns of the resist. The spacing between microlenses can be made narrower than the spacing between the planar patterns of the resist. Even when the planar shape of the microlens is an ellipse, the curvatures can be optimized in both the row and column directions by making the heights in these directions different from each other. It is possible to provide a microlens array having a small non-focusing region and a solid-state image pickup device having a high sensitivity and little smear.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.08/921,629 filed Aug. 27, 1997, allowed Attorney Docket No. P97,1373.The present and foregoing application claim priority to Japaneseapplications Nos. P08-249244 filed May 16, 1997, and P08-271798 filedMay 16, 1997. The foregoing applications are incorporated herein byreference to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microlens array in which a pluralityof microlenses are arranged, a method of forming the microlens array, asolid-state image pickup device having the microlens array, and a methodof manufacturing the solid-state image pickup device.

2. Description of the Related Art

To realize, for example, a solid-state image pickup device which formsfine images and is small in size and light in weight, it is necessary todecrease the area of each pixel. As a consequence, however, the incidentlight quantity per pixel decreases, and this lowers the sensitivity.Therefore, a microlens array having microlenses in a one-to-onecorrespondence with a plurality of photosensitive portions is providedin an on-chip state. Each microlens focuses, toward a photosensitiveportion, even light incident on a portion except the photosensitiveportion.

FIGS. 1A to 2 show the first related art of a CCD solid-state imagepickup device having this microlens array and a method of manufacturingthis CCD solid-state image pickup device. In the manufacturing method ofthis first related art, as shown in FIG. 1A, photosensitive portions 12are formed in a semiconductor substrate 11, and conductive films 13 usedto isolate pixels and as charge transfer electrodes are formed on thesemiconductor substrate 11. The conductive films 13 and the like arecovered with light-shielding films 14, and the light-shielding films 14are covered with a planarizing film 15. In addition, a color filter 16is formed on the planarizing film 15.

Thereafter, a material layer 17 of a microlens array is formed on thecolor filter 16 by using, e.g., a polystyrene resin or a polyimideresin. The surface of the material layer 17 is coated with a resist 21,and the resist 21 is processed into planar patterns of the microlensarray. As shown in FIG. 1B, the resist 21 is formed into athree-dimensional shape of the microlens array by reflow.

In FIG. 1C, the resist 21 and the material layer 17 are simultaneouslyetched to transfer the three-dimensional shape of the resist 21 to thematerial layer 17. Consequently, as shown in FIG. 2, a microlens array23 having microlenses 22 in a one-to-one correspondence with thephotosensitive portions 12 is formed.

Note that when the resist 21 and the material layer 17 aresimultaneously etched, the material layer 17 is formerly side-etched toproduce a negative critical dimension or CD loss by which the planarpatterns transferred from the resist 21 to the material layer 17 aresmaller than the planar patterns of the resist 21. Recently, however,the planar patterns of the resist 21 are accurately transferred to thematerial layer 17.

When the resist 21 is processed in the step shown in FIG. 1A, a spacingx of about 0.4 m is formed between the planar patterns of the resist 21due to the limit of the resolution of lithography. This spacing x can benarrowed by reflow in the step shown in FIG. 1B. However, if the planarpatterns of the resist 21 are made too close to each other, the planarpatterns of the resist 21 can contact each other due to, e.g., avariation in reflow.

If the planar patterns of the resist 21 contact each other, the resist21 is planarized by, e.g., the surface tension of the resist 21, andthis makes the formation of the microlens array 23 impossible.Therefore, even after reflow, the spacing between the planar patterns ofthe resist 21 can be narrowed to at most about 0.3 μm.

FIGS. 3A and 3B show vertical and horizontal sections of the secondrelated art of a solid-state image pickup device having a microlensarray. In this second related art, photosensitive portions 32 are formedin a semiconductor substrate 31, and conductive films 33 to be used toisolate pixels and as charge transfer electrodes are formed on thesemiconductor substrate 31. The conductive films 33 and the like arecovered with light-shielding films 34, and the light-shielding films 34and the like are covered with a planarizing film 35.

A color filer 36 is formed on the planarizing film 35, and microlenses37 are formed on this color filter 36. The photosensitive portions 32are arranged in a matrix manner, and the microlenses 37 are formed in aone-to-one correspondence with these photosensitive portions 32. Asshown in FIG. 4, the microlenses 37 are also arranged in a matrix mannerto form a microlens array 38.

In the first related art shown in FIGS. 1A to 2, the planar patterns ofthe resist 21 are accurately transferred to the material layer 17.Therefore, as shown in FIG. 2, the spacing between the microlenses 22 isalso as wide as about 0.3 μm, so only the microlens array 23 having alarge non-focusing region can be formed.

In the microlens array 23 having this large non-focusing region,however, as can be seen from FIG. 2, incident light 24 is noteffectively focused on the photosensitive portions 12. Additionally, alarge quantity of the incident light 24 is obliquely incident on thephotosensitive portions 12 by being reflected by the light-shieldingfilms 14 or the like and enters into portions below the charge transferelectrodes. Accordingly, in this first related art it is difficult toaccomplish a solid-state image pickup device with a high sensitivity andlittle smear.

The pattern of each pixel of a solid-state image pickup device isgenerally a rectangle rather than a square. Therefore, as shown in FIG.4, the planar shape of the microlens 37 is also an ellipse or a shapeclose to an ellipse, rather than a true circle. Nevertheless, in thesecond related art described above, the microlens 37 has equal heights yin both the vertical and horizontal directions regardless of whether thediameter of the microlens 37 is small as shown in FIG. 3A or large asshown in FIG. 3B.

Accordingly, the curvature of the microlens 37 in the vertical directiondiffers from that in the horizontal direction. When the focal point ofincident light 39 is positioned in the photosensitive portion 32 in thehorizontal direction, the focal point of the incident light 39 is notpositioned in the photosensitive portion 32 in the vertical direction.Consequently, in the above second related art, the sensitivity is notnecessarily sufficiently high although the microlenses 37 are used, andit is difficult to reduce smear or the like. This phenomenon becomestypical as the aspect ratio of the pattern of each pixel increases.

Note that it is being attempted to adjust curvatures by making lensheights in the vertical and horizontal directions substantiallydifferent from each other by forming a cylindrical lens continuouslyextending in the vertical or horizontal direction and further formingseparate lenses across this cylindrical lens in a one-to-onecorrespondence with pixels.

Unfortunately, it is presently very difficult to form two types oflenses overlapped as above. Also, when such two types of lenses areformed, the number of manufacturing steps increases to lower theproductivity, and this increases the manufacturing cost of a solid-stateimage pickup device. Additionally, even when such two types of lensesare used, interfaces exist inside the lenses so incident light isreflected more. Accordingly, it is still difficult to increase thesensitivity and reduce smear or the like.

OBJECT AND SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide amicrolens array in which the spacing between microlenses is narrow and anon-focusing region is small, and a solid-state image pickup devicehaving a high sensitivity and little smear.

According to one aspect of the invention, in a microlens array, theheights of a microlens in the row and column directions are differentfrom each other and are made from a single material layer. For thisreason, the curvatures can be optimized in both the row and columndirections by adjusting these heights. Consequently, even when theplanar shape of the microlens is not a true circle, the focal points ofincident light in the row and column directions can be positioned in asingle point. Additionally, since the microlens is made from a singlematerial layer, no interface exists inside the microlens, so incidentlight is reflected little. Accordingly, it is possible to realize asolid-state image pickup device with a high sensitivity and littlesmear, a liquid crystal display device which displays clear images orthe like.

According to another aspect of the invention, in a microlens array, theplanar shape of a microlens is an ellipse. For this reason, incidentlight can be effectively focused by opposing the microlens to a wideportion of even a region having different lengths in the row and columndirections. Accordingly, although the degree of freedom of the patternof each pixel is high, it is possible to realize a solid-state imagepickup device with a high sensitivity and little smear, a liquid crystaldisplay device which displays clear images or the like.

According to another aspect of the invention, a microlens array opposesa plurality of photosensitive portions of a solid-state image pickupdevice. For this reason, external incident light can be effectivelyfocused on these photosensitive portions even when the pattern of eachpixel of the solid-state image pickup device is a rectangle.Accordingly, a solid-state image pickup device with a high sensitivityand little smear can be realized although the degree of freedom of thepattern of each pixel is high.

According to another aspect of the invention, a microlens arrayaccording to claim 4 opposes a plurality of display portions of a liquidcrystal display device. For this reason, incident light from thesedisplay portions can be effectively focused even when the pattern ofeach pixel of the liquid crystal display device is a rectangle.Accordingly, a liquid crystal display device which displays clear imagescan be realized although the degree of freedom of the pattern of eachpixel is high.

In a method of forming a microlens array according to an aspect of theinvention, a mask layer and a material layer of a microlens array aresimultaneously etched under a condition by which planar patternstransferred from the mask layer to the material layer are larger thanplanar patterns of the mask layer. For this reason, a microlens arraywith a small non-focusing region can be formed by making the spacingbetween microlenses narrower than the spacing between the planarpatterns of the mask layer. Accordingly, it is possible to realize asolid-state image pickup device with a high sensitivity and littlesmear, a liquid crystal display device which displays clear images orthe like.

In a method of forming a microlens array according to another aspect ofthe invention, after microlenses start contacting each other in adirection in which the spacing between dome-like portions of a masklayer is narrow, the height of the microlenses in this direction becomessubstantially smaller than the height in a direction perpendicular tothat direction. Therefore, the curvatures can be optimized in both therow and column directions by adjusting the heights of the microlenses byadjusting the spacings between the dome-like portions of the mask layer.

As a consequence, it is possible to form a microlens by which the focalpoints of incident light in the row and column directions are positionedin a single point, even when the planar shape of the microlens is not atrue circle. Additionally, since the material layer of the microlensescan be a single layer, a microlens which has no interface inside it andrarely reflects incident light can be formed. Accordingly, it ispossible to realize a solid-state image pickup device with a highsensitivity and little smear, a liquid crystal display device whichdisplays clear images or the like.

In a method of forming a microlens array according to another aspect ofthe invention, etching is performed by using a resist as a mask layer, agas mixture of oxygen and CF₄ or CF₄ as an etching gas, and a magnetronRIE system. For this reason, it is possible to produce a positivecritical dimension by which planar patterns transferred from the resistas a mask layer to a material layer of a microlens array are larger thanplanar patterns of the resist. Additionally, the amount of this positivecritical dimension can be controlled by properly selecting the volumemixing ratio of oxygen to CF₄ as an etching gas. Accordingly, it isreadily possible to accomplish a solid-state image pickup device with ahigh sensitivity and little smear, a liquid crystal display device whichdisplays clear images or the like.

In a method of forming a microlens array according to another aspect ofthe invention, the flow rate of an etching gas is set at 10 to 100 ccm.This produces a positive critical dimension by which planar patternstransferred from a resist as a mask layer to a material layer of amicrolens array are larger than planar patterns of the resist.Accordingly, it is possible to accomplish a solid-state image pickupdevice with a high sensitivity and little smear, a liquid crystaldisplay device which displays clear images or the like.

In a method of forming a microlens array according to another aspect ofthe invention, the volume mixing ratio of oxygen to CF₄ in a gas mixtureas an etching gas is set at 1:1 to 10. This produces a positive criticaldimension by which planar patterns transferred from a resist as a masklayer to a material layer of a microlens array are larger than planarpatterns of the resist. Accordingly, it is possible to accomplish asolid-state image pickup device with a high sensitivity and littlesmear, a liquid crystal display device which displays clear images orthe like.

In a method of forming a microlens array according to another aspect ofthe invention, a radio frequency power generated by a magnetron RIEsystem is set at 1.0 to 8.0 W/cm². This produces a positive criticaldimension by which planar patterns transferred from a resist as a masklayer to a material layer of a microlens array are larger than planarpatterns of the resist. Accordingly, it is possible to realize asolid-state image pickup device with a high sensitivity and littlesmear, a liquid crystal display device which displays clear images orthe like.

In a method of forming a microlens array according to another aspect ofthe invention, the internal pressure of an etching chamber in amagnetron RIE system is set at 1.3 to 13.3 Pa. This produces a positivecritical dimension by which planar patterns transferred from a resist asa mask layer to a material layer of a microlens array are larger thanplanar patterns of the resist. Accordingly, it is possible to realize asolid-state image pickup device with a high sensitivity and littlesmear, a liquid crystal display device which displays clear images orthe like.

In a method of forming a microlens array according to another aspect ofthe invention, the planar shape of a mask layer is an ellipse. For thisreason, it is possible to form a microlens array in which incident lightcan be effectively focused by opposing a microlens to a wide portion ofeven a region having different lengths in the row and column directions.Accordingly, although the degree of freedom of the pattern of each pixelis high, it is possible to realize a solid-state image pickup devicewith a high sensitivity and little smear, a liquid crystal displaydevice which displays clear images or the like.

In a method of forming a microlens array according to another aspect ofthe invention, a microlens array is so formed as to oppose a pluralityof photosensitive portions of a solid-state image pickup device. Forthis reason, it is possible to form a microlens array in which externalincident light can be effectively focused on these photosensitiveportions even when the pattern of each pixel of the solid-state imagepickup device is a rectangle. Accordingly, a solid-state image pickupdevice with a high sensitivity and little smear can be realized althoughthe degree of freedom of the pattern of each pixel is high.

In a method of forming a microlens array according to another aspect ofthe invention, a microlens array is so formed as to oppose a pluralityof display portions of a liquid crystal display device. For this reason,it is possible to form a microlens array in which incident light fromthese display portions can be effectively focused even when the patternof each pixel of the liquid crystal display device is a rectangle.Accordingly, a liquid crystal display device which displays clear imagescan be realized although the degree of freedom of the pattern of eachpixel is high.

In a solid-state image pickup device according to another aspect of theinvention, the heights of a microlens in the row and column directionsare different from each other. For this reason, the curvatures can beoptimized in both the row and column directions by adjusting theseheights. Consequently, even when the planar shape of the microlens isnot a true circle, the focal points of incident light in the row andcolumn directions can be positioned in a photosensitive portion.Additionally, since the microlens is made from a single material layer,no interface exists inside the microlens, so incident light is rarelyreflected. Accordingly, the sensitivity is high and smear is little.

In a solid-state image pickup device according to another aspect of theinvention, the planar shape of a microlens is an ellipse. For thisreason, incident light can be effectively focused by opposing themicrolens to a wide portion of even a pixel having different lengths inthe row and column directions. Accordingly, the sensitivity is high andsmear is little although the degree of freedom of the pattern of eachpixel is high.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, a mask layer and a material layer ofa microlens array are simultaneously etched under a condition by whichplanar patterns transferred from the mask layer to the material layerare larger than planar patterns of the mask layer. For this reason, amicrolens array with a small non-focusing region can be formed by makingthe spacing between microlenses narrower than the spacing between theplanar patterns of the mask layer. Accordingly, a solid-state imagepickup device with a high sensitivity and little smear can bemanufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, after microlenses start contactingeach other in a direction in which the spacing between dome-likeportions of a mask layer is narrow, the height of the microlenses inthis direction becomes substantially smaller than the height in adirection perpendicular to that direction. Therefore, the curvatures canbe optimized in both the row and column directions by adjusting theheights of the microlenses by adjusting the spacings between thedome-like portions of the mask layer.

As a consequence, it is possible to form a condenser microlens by whichthe focal points of incident light in the row and column directions arepositioned in a photosensitive portion, even when the planar shape ofthe microlens is not a true circle. Additionally, since the materiallayer of the microlenses can be a single layer, a microlens which has nointerface inside it and rarely reflects incident light can be formed.Accordingly, a solid-state image pickup device with a high sensitivityand little smear can be manufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, the planar shape of a mask layer isan ellipse. For this reason, it is possible to form a microlens array bywhich incident light can be effectively focused by opposing themicrolens to a wide portion of even a pixel having different lengths inthe row and column directions. Accordingly, a solid-state image pickupdevice with a high sensitivity and little smear can be manufacturedalthough the degree of freedom of the pattern of each pixel is high.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, etching is performed by using aresist as a mask layer, a gas mixture of oxygen and CF₄ or CF₄ as anetching gas, and a magnetron RIE system. For this reason, it is possibleto produce a positive critical dimension by which planar patternstransferred from the resist as a mask layer to a material layer of amicrolens array are larger than planar patterns of the resist.Additionally, the amount of this positive critical dimension can becontrolled by properly selecting the volume mixing ratio of oxygen toCF₄ as an etching gas. Accordingly, a solid-state image pickup devicewith a high sensitivity and little smear can be easily manufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, the flow rate of an etching gas isset at 10 to 100 ccm. This produces a positive critical dimension bywhich planar patterns transferred from a resist as a mask layer to amaterial layer of a microlens array are larger than planar patterns ofthe resist. Accordingly, a solid-state image pickup device with a highsensitivity and little smear can be manufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, the volume mixing ratio of oxygen toCF₄ in a gas mixture as an etching gas is set at 1:1 to 10. Thisproduces a positive critical dimension by which planar patternstransferred from a resist as a mask layer to a material layer of amicrolens array are larger than planar patterns of the resist.Accordingly, a solid-state image pickup device with a high sensitivityand little smear can be manufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, a gas mixture containing a largeramount of CF₄ than oxygen is used as an etching gas. This readilyproduces a positive critical dimension by which planar patternstransferred from a resist as a mask layer to a material layer of amicrolens array are larger than planar patterns of the resist.Accordingly, a solid-state image pickup device with a high sensitivityand little smear can be easily manufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, a radio frequency power generated bya magnetron RIE system is set at 1.0 to 8.0 W/cm². This produces apositive critical dimension by which planar patterns transferred from aresist as a mask layer to a material layer of a microlens array arelarger than planar patterns of the resist. Accordingly, a solid-stateimage pickup device with a high sensitivity and little smear can bemanufactured.

In a method of manufacturing a solid-state image pickup device accordingto another aspect of the invention, the internal pressure of an etchingchamber in a magnetron RIE system is set at 1.3 to 13.3 Pa. Thisproduces a positive critical dimension by which planar patternstransferred from a resist as a mask layer to a material layer of amicrolens array are larger than planar patterns of the resist.Accordingly, a solid-state image pickup device with a high sensitivityand little smear can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are side sectional views showing a method ofmanufacturing a CCD solid-state image pickup device having a microlensarray in order of steps;

FIG. 2 is a side sectional view of a CCD solid-state image pickup deviceaccording to the first related art of the present invention;

FIGS. 3A and 3B are side sectional views of a CCD solid-state imagepickup device according to the second related art of the presentinvention, in each of which the left half and the right half showportions taken along lines A--A and B--B, respectively, in FIG. 4, inwhich FIG. 3A shows a case where the diameter of a condenser microlensis relatively small and FIG. 3B shows a case where the diameter of thecondenser microlens is relatively large;

FIG. 4 is a plan view of a condenser microlens array in a CCDsolid-state image pickup device to which the present invention isapplicable;

FIG. 5 is a side sectional view of a CCD solid-state image pickup deviceaccording to the first embodiment of the present invention;

FIG. 6 is a side sectional view of a CCD solid-state image pickup deviceaccording to the second embodiment of the present invention, in whichthe left half and the right half show portions taken along the linesA--A and B--B, respectively, in FIG. 4;

FIGS. 7A and 7B are side sectional views showing the first half of amanufacturing method according to the second embodiment in order ofsteps; and

FIGS. 8A and 8B are side sectional views showing the second half of themanufacturing method according to the second embodiment in order ofsteps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first and second embodiments of the present invention applied to aCCD solid-state image pickup device and a method of manufacturing thesame will be described below with reference to FIGS. 5 to 8. FIG. 5shows the first embodiment. A manufacturing method of this firstembodiment is substantially the same as the manufacturing method of thefirst related art described earlier until the step shown in FIG. 1B andis also the same as the first related art in that a resist 21 and amaterial layer 17 are simultaneously etched as shown in FIG. 1C.

The first embodiment, however, differs from the first related art in theetching conditions. That is, the first embodiment uses a magnetronetching RIE system for generating a radio frequency power of 1.0 to 8.0W/cm² for etching. Also, a gas mixture of oxygen and CF₄ mixed at avolume mixing ratio of 1:1 to 10 or CF₄ alone is supplied as an etchinggas at a flow rate of 10 to 100 ccm, and the internal pressure of anetching chamber is set at 1.3 to 13.3 Pa.

When etching is performed under the above conditions, the plasma densityis high and a large amount of deposition product is formed in themagnetron RIE system. Consequently, the etching of the resist 21 and thematerial layer 17 and the deposition of the product progress at the sametime. However, while the deposition uniformly progresses on the entiresurface, the etching rate is high near the tops of the resist 21 and thematerial layer 17 because the ion assist effect is notable in theseregions and is low near the trenches because the ion assist effect isinsignificant in these regions.

As a consequence, a positive critical dimension or CD gain by whichplanar patterns transferred from the resist 21 to the material layer 17become larger than planar patterns of the resist 21 is produced. Theamount of this CD gain can be controlled within the range of 0.1 to 1.0μm by properly selecting the volume mixing ratio of oxygen and CF₄ as anetching gas.

In the first embodiment as described above, even when a spacing of about0.3 μm is present between the planar patterns of the resist 21 as shownin FIG. 1C, a microlens array 26 in which almost no non-focusing regionexists between microlenses 25 as shown in FIG. 5 can be formed.

In the microlens array 26 having almost no non-focusing region, as canbe seen from FIG. 5, not only incident light 24 is effectively focusedon photosensitive portions 12 but also the amount of the incident light24 which obliquely is incident on the photosensitive portions 12 bybeing reflected by light-shielding films 14 or the like and enters intoportions below charge transfer electrodes is small. For these reasons,the CCD solid-state image pickup device in FIG. 5 according to the firstembodiment has a sensitivity higher by 20-odd % and has less smear thanthe CCD solid-state image pickup device in FIG. 2 according to the firstrelated art.

In the above first embodiment, a magnetron RIE system is used to etchthe resist 21 and the material layer 17. However, it is also possible touse a parallel plate RIE system, a high-pressure, narrow-gap plasmaetching system, an ECR etching system, a microwave plasma etchingsystem, or other high-density plasma etching systems such as atransformer coupled plasma etching system, an inductively coupled plasmaetching system and a helicon wave plasma etching system.

Also, in the above first embodiment, an etching gas containing CF₄ isused to etch the resist 21 and the material layer 17. However, it ispossible to use, instead of CF₄, flon gases such as C₂ F₆, C₃ F₈, C₄ F₈,CHF₃ and CH₂ F₂, halogen gases such as Cl₂, HCl, HBr and BCl₃, andnitrogen and oxide gases such as N₂, CO and CO₂.

FIGS. 6 to 8 show the second embodiment. In a manufacturing method ofthis second embodiment, steps substantially the same as in themanufacture of the second related art shown in FIG. 3 are executed untila material layer 41 of a microlens array is formed on a color filter 36by using a polystyrene resin, a polyimide resin or the like as shown inFIG. 7A.

Thereafter, a resist 42 is processed into matrix patterns for formingcondenser microlenses on the color filter 36. In this processing, aspacing a between the resists 42 is made relatively narrow in adirection in which the height of the microlenses is to be decreased anda spacing b between the resists 42 is made relatively wide in adirection in which the height of the microlenses is to be increased.

Subsequently, as shown in FIG. 7B, the resists 42 are deformed into theform of a dome by reflow. Even after this deformation, a spacing cbetween the resists 42 in the direction in which the height of themicrolenses is to be decreased is relatively narrow, and a spacing dbetween the resists 42 in the direction in which the height of themicrolenses is to be increased is relatively wide.

As shown in FIG. 8A, by using a magnetron RIE system for generating aradio frequency power of 1.0 to 8.0 W/cm², the resists 42 and thematerial layer 41 are simultaneously etched under a condition by whichthe etching selectivity of the resists 42 and the material layer 41 isclose to 1, while an etching gas 43 which is a gas mixture of oxygen andCF₄ mixed at a volume mixing ratio of 1:1 to 10 or CF₄ alone is suppliedat a flow rate of 10 to 100 ccm and the internal pressure of an etchingchamber is set at 1.3 to 13.3 Pa. During the etching, CF₄ sequentiallydissociates as follows.

    CF.sub.4 CF+F

    CF.sub.3 CF+F

    CF.sub.2 CF+F

Of the dissociation products of the etching gas 43, O and F are etchingspecies, and CF and CF₂ are deposition species. Etching and depositionare competitively advanced by these species.

A portion near the base of the dome-like resist 42 is nearly vertical.Therefore, this portion is not easily sputtered by O⁺ ion, so a depositoriginally readily adheres to this portion. Additionally, a magnetronRIE system uses a magnetic field, so electrons violently move. It istherefore considered that the plasma dissociation efficiency is high andthe production amounts of CF and CF₂ as deposition species are large.

As a consequence, reposition is superior to etching in a portion nearthe base of the dome-like resist 42. This produces a positive criticaldimension by which patterns are gradually enlarged as the dome-likeshapes are transferred from the resists 42 to the material layer 41 byetching. As shown in FIG. 8B, therefore, although a flat portion iseliminated from the material layer 41 in the direction in which thedome-like resists 42 are arranged at the narrow spacing c, a flatportion remains in the material layer 41 in the direction in which theresists 42 are arranged at the wide spacing d.

Etching by the magnetron RIE system is further continued from the stateshown in FIG. 8B to form a microlens array 45, in which microlenses 44are arranged in a matrix manner, from the material layer 41 as shown inFIG. 6. During the formation, dome-like portions of the material layer41 start contacting each other to bury trenches between the microlenses44 earlier in the direction in which the dome-like resists 42 arearranged at the narrow spacing c.

Consequently, a height e from the surface of the color filter 36 to thebase of the microlens 44 in the direction in which the dome-like resists42 are arranged at the narrow spacing c is larger than a height f in thedirection in which the resists 42 are arranged at the spacing d. Thatis, a height g of the microlenses 44 in the direction in which thedome-like resists 42 are arranged at the narrow spacing c is smallerthan a height h in the direction in which the resists 42 are arranged atthe wide spacing d.

Accordingly, the curvatures can be optimized by adjusting the heights gand h of the microlenses 44 in the row and column directions byadjusting the spacings c and d between the dome-like resists 42.Consequently, although the planar shape of the microlens 44 is anellipse, the focal points of incident light can be positioned in thephotosensitive portion 32 in both the row and column directions.Additionally, since the microlens 44 has no interface inside it andrarely reflects incident light, the CCD solid-state image pickup deviceof this second embodiment has a high sensitivity and little smear.

The above second embodiment uses the etching gas 43 which is a gasmixture of oxygen and CF₄ mixed at a volume mixing ratio of 1:1 to 10 orCF₄ alone. However, when the etching gas 43 in which the ratio of CF₄ ishigh, e.g., a gas mixture of oxygen and CF₄ mixed at a volume mixingratio of 1:10 or CF₄ alone is used, the microlenses 44 having thedifferent heights g and h in the row and column directions can be easilyformed.

Also, in the second embodiment, etching is continued until themicrolenses 44 contact each other in both the row and column directionsas shown in FIG. 6. However, when the microlenses 44 start contactingeach other in one direction, the heights of the microlenses 44 in therow and column directions become different from each other. Therefore,etching can be stopped at that timing although the microlenses 44 havenot started contacting each other in the other direction.

Furthermore, the second embodiment uses a magnetron RIE system and theetching gas 43 which is a gas mixture of oxygen and CF₄ mixed at avolume mixing ratio of 1:1 to 10 or CF₄ alone. However, another etchingsystem or etching gas can also be used as long as it is possible toobtain conditions which produce a positive critical dimension by whichpatterns are gradually enlarged as the dome-like shapes are transferredfrom the resists 42 to the material layer 41 by etching.

Moreover, in the first and second embodiments described above thepresent invention is applied to a CCD solid-state image pickup deviceand a method of manufacturing the same. However, the present inventionis also applicable to the manufacture of another solid-state imagepickup device such as a MOS solid-state image pickup device forperforming amplification or a liquid crystal display device, providedthat a microlens array is formed in an on-chip state.

What is claimed is:
 1. A method of forming a microlens array, comprisingthe steps of:forming a mask layer having dome-like three-dimensionalshapes of microlenses of a microlens array on a material layer of saidmicrolens array; and simultaneously etching said mask layer and saidmaterial layer under a condition by which planar patterns transferredfrom said mask layer to said material layer are larger than planarpatterns of said mask layers, until said microlenses start contactingeach other.
 2. A method of forming a microlens array, comprising thesteps of:forming a mask layer having a three-dimensional shapes of amicrolens array on a material layer of said microlens array; andsimultaneously etching said mask layer and said material layer under acondition by which planar patterns transferred from said mask layer tosaid material layer are larger than planar patterns of said mask layers,wherein,said mask layer has dome-like portions arranged in a matrixmanner, and the etching is performed until said microlenses startcontacting each other, a resist is used as said mask layer, a gasmixture of oxygen and CF₄ or CF₄ is used as an etching gas, and theetching is performed by using a magnetron RIE system.
 3. A methodaccording to claim 1, whereina resist is used as said mask layer, a gasmixture of oxygen and CF₄ or CF₄ is used as an etching gas, and theetching is performed by using a magnetron RIE system.
 4. A methodaccording to claim 3, wherein a flow rate of said etching gas is 10 to100 ccm.
 5. A method according to claim 3, wherein a volume mixing ratioof oxygen to CF₄ in said gas mixture is 1:1 to
 10. 6. A method accordingto claim 3, wherein a radio frequency power generated by said magnetronRIE system is 1.0 to 8.0 W/cm².
 7. A method according to claim 3,wherein an internal pressure of an etching chamber of said magnetron RIEsystem is 1.3 to 13.3 Pa.
 8. A method according to claim 1, wherein aplanar shape of said mask layer is an ellipse.
 9. A method according toclaim 1, wherein said microlens array is formed to oppose a plurality ofphotosensitive portions of a solid-state image pickup device.
 10. Amethod according to claim 1, wherein said microlens array is formed tooppose a plurality of display portions of a liquid crystal displaydevice.
 11. A method of manufacturing a solid-state image pickup devicehaving a microlens array in which a plurality of condenser microlensesare arranged in a matrix manner in correspondence with a plurality ofphotosensitive portions arranged in a matrix manner, comprising thesteps of:forming a mask layer having a three-dimensional shape of saidmicrolens array on a material layer of said microlens array; andsimultaneously etching said mask layer and said material layer under acondition by which planar patterns transferred from said mask layer tosaid material layer are larger than planar patterns of said mask layer,wherein,said mask layer has dome-like portions arranged in a matrixmanner at spacings in row and column directions, and the etching isperformed until said microlenses start contacting each other.